1. Field of the Invention
This invention relates to a method and apparatus for continuous casting and more particularly to a method and apparatus for continuous casting in which molten metal is continuously fed into a cooled cylindrical mold where a cast section is formed by allowing the molten metal to start solidification below the surface thereof and the formed cast section is then withdrawn from the mold.
The method and apparatus of this invention are applicable to the continuous casting of billets and other shapes of carbon steels, stainless steels and other metals.
2. Description of the Prior Art
Horizontal continuous casting is one of the known processes that solidifies molten metal continuously fed to a cooled cylindrical mold below the surface of the molten metal. In horizontal continuous casting, a break ring provided at the inlet of the mold stabilizes the start of metal solidification. The break ring has a circumferential step protruding into the mold whose inside diameter is larger than that of the step. To keep the break ring in close contact with the mold, for example, their mating surfaces are tapered and pressed against each other.
Solidification of the molten metal in the mold starts in a region close to the periphery of the forward end of the break ring (which is a downstream portion of the metal stream), with the solidified shell growing while being intermittently withdrawn through the exit end of the mold.
Gas bubbles often form in a subsurface portion of the sections cast by the above method. There are several reasons for this. Even when the break ring is pressed against the mold as described above, for example, a gap can result from thermal expansion or other causes. No air is admitted to near the break ring that allows metal solidification to start below the surface of the molten metal because the ferrostatic pressure of the molten metal at the break ring where solidification starts is higher than atmospheric. When the solidified shell is withdrawn and detached from the forward end of the break ring, however, a nearly evacuated gap forms between the forward end of the break ring and the rear end of the solidified shell (that faces the forward end of the break ring), though only for a short period of time. The air then passes from outside the break ring, through an opening between the mating surfaces of the break ring and the mold, to that gap and further into the molten metal to form gas bubbles. Sometimes, the air admitted from the exit end of the mold passes through the opening between the break ring and the mold to that gap and into the molten metal to form gas bubbles.
The gas bubbles form in a region 2 mm to 3 mm below the surface of the cast section. When subsequently rolled, the gas bubbles in cast sections result in various types of surface defects, such as seams and longitudinal cracks. The defects thus formed are particularly serious with stainless steels and other products that must meet stringent surface quality requirements. Therefore, the gas bubbles must be removed by scarfing or other surface conditioning processes, which, however, add to production costs and lower production yield.
In the Japanese Provisional Utility Model Publication No. 38136 of 1989 is disclosed technology for fitting a break ring in such a manner as to prevent the infiltration of the air. This technology hermetically seals the junction where a break ring and a molten metal cooling segment (a mold) meet with an annular gasket of a heat-resistant material. But the annular gasket deteriorates when it is heated, for example, by the heat from the mold to above the temperature it can withstand. The damaged annular gasket loses its sealing function, with resultant infiltration of the air into the mold and formation of gas bubbles in the solidified shell.
The U.S. Pat. No. 4,817,701 discloses continuous casting technology that seals a molten metal feed nozzle and the inlet of a mold with an inert gas that does not react with molten metal. The object of this technology is to completely prevent the infiltration of gases in the atmosphere that oxidize the surface of molten metal. But this technology too is not quite free of the risk of forming gas bubbles in cast sections.
By analyzing the gas contained in the formed bubbles to determine the cause of their formation, the inventors learned that the gas in the bubbles consisted mainly of argon and the metal surrounding the bubbles showed a higher nitrogen content than elsewhere. From this finding it was presumed that nitrogen in the air dissolved in molten metal, but argon, which is insoluble in molten metal, remained intact as gas bubbles. To confirm this presumption, a continuous casting test was performed by supplying an inert argon gas to inside a shielding means that surrounds the periphery of the break ring as in the technology of the U.S. patent mentioned before. In the test, more gas bubbles were formed in the subsurface region of the cast sections than in the conventional argon-free continuous casting operation. No gas bubbles were formed when nitrogen, which is soluble in molten metal, was supplied in place of insoluble argon. The present invention is based on the finding just described.
Japanese Provisional Patent Publication No. 71157 of 1986 discloses horizontal continuous casting technology using a cylindrical mold in which nitrogen is supplied to a portion of a corner member, which consists of a refractory plate projecting inward from the inner surface of the lindrical mold, that lies below the axis of cylindrical mold. This technology uniformly cools the entire surface of the solidified shell by shifting downstream the point where molten metal comes in contact with the inner surface of the lower portion of the mold. Introducing nitrogen only to the lower portion of the corner member, however, does not prevent the infiltration of the air into the mold through the entire circumference of the junction where the break ring meets the inner surface of the mold.